“Did Life Evolve in Ice?”
by Douglas Fox
Discover, February 2008
Was ice the cradle of life?
In 1953, Stanley Miller first created the building blocks of life in his laboratory by simulating lightning on early Earth. His final experiment, 50 years later, helped open a surprising new chapter in the search for life’s origins.
One morning in April 1999, Stanley Miller lifted a glass vial from a bubbling vat. For 25 years, he had cared for that vial as though it were a pet kitten, checking on it each morning, adding a few pellets of dry ice to keep it frozen, at –108 ºF. He had told hardly a soul of its existence.
With wrinkled, 75-year-old hands, Miller finally opened his time capsule and ended the experiment that had lasted a third of his life. The contents of that vial would change the world.
Miller had filled the vial, in the 1970s, with a primordial mix of ammonium and cyanide. He then cooled it to the temperature of Jupiter’s icy moon Europa—too cold, most scientists had assumed, for much to happen. But in 25 years, Miller’s concoction had coalesced into the stuff of life: building blocks of RNA, DNA, and proteins.
Speculation of life’s origin has long fixated on hydrothermal vents, volcanoes, and heat-loving bacteria. But Miller’s secret experiment catapulted a new theory onto center stage: that life began not in soup, but in primordial glaciers or sea ice. A steady stream of new evidence is supporting the idea.
RNA molecules—which acted as the first genes and enzymes—are notoriously fragile at room temperature, let alone hot volcanic vents. But in ice, RNA can survive for centuries rather than days— long enough, perhaps, to spawn self-replicating molecules. Recent experiments in artificial sea ice produced RNA chains up to 700 nucleotides long—several times longer than standard primordial soup experiments have ever produced (these new results will be published in 2007).
The first RNA enzymes were inefficient, making it difficult to elongate RNA molecules faster than they fell apart. But ice helped here, too. Researchers have recently used reverse evolution to create progressively smaller, fragmented, and more mutated versions of an RNA enzyme called HPR, which
joins RNA molecules together to make longer chains. These primitive versions of HPR couldn’t stitch RNA chains together at room temperature, but in ice they were surprisingly active. So ice might have held fragile life together until it became efficient enough to survive in warmer places.
It squares with the fact that early Earth’s oceans were probably frozen (since the young sun was dimmer than today). It also suggests that icy havens on Mars, Europa, Titan, or Enceladus could have spawned life.
Although life requires liquid water, small amounts of liquid can exist even 150 degrees F below freezing. Ice’s hidden environments—microscopic channels and nanofilms of liquid—may have spawned life by gathering and organizing key molecules on its surfaces. A cubic yard of sea ice contains billions of liquid compartments—microscopic test tubes (perhaps, the first cells) that each created unique mixtures of RNA.
Even today, liquid oases in ice provide havens for life on Earth: microbes that survive a million years beneath kilometers of glacial ice. Some of these bacteria live and breathe within a film of liquid water just one molecule thick. (The microcosm of ice provides one of several options for photographs.)
Life has also shown a tremendous capacity for modifying ice to its liking. Every year, microbes in the Arctic Ocean ooze out 100 million tons of goopy chemicals that change the structure of sea ice, creating networks of tunnels where life can flourish. Such could be the case in other worlds—providing an obvious clue for space probes to find. This story will also document ExoMars and other space missions that will search for life.